Many of ultrasonic transducers currently used in ultrasonic probes or the like transmit and receive ultrasound waves by utilizing a piezoelectric effect and a inverse piezoelectric effect of ceramic-based piezoelectric substances such as PZT (lead zirconate titanate).
In most of the ultrasonic transducers, PZT elements have been used heretofore as oscillators for the transducers. In order to provide, as an alternative of these piezoelectric type transducers, a transducer array which is more efficient and which supports a broader bandwidth, capacitive micromachined ultrasonic transducers (cMUT) have been studied a lot recently. The cMUTs are manufactured by using a microfabrication technique in which the silicon surfaces and bulks are machined by microns by standard silicon processing techniques. The cMUT generally has a structure in which a periphery of a vibrating membrane of microscopic size (e.g., having a diameter of 50 μm) is fixed by supporting posts. The cMUT functions as an electro-acoustic transducer by applying voltage to electrodes provided in the vibrating membrane and in the lower side substrate.
As is obvious also from the structure of the cMUT, the supporting strength for the cMUT membrane portion and the mechanical/electrical/structural boundary conditions of the area around the membrane portion determine the ultimate structure of the membrane portion. Variation among the ultimate structures gives a large impact on the operation reliability of the cMUTs as ultrasonic transducers and the acoustic characteristics such as the transmission-reception sensitivity of transducer. Patent Document 1 discloses a technique for reducing a residual stress of an insulating film for a cMUT upper side electrode by doping a stress reducing agent such as germanium into a silicon layer and the like.
Patent Document 1: Japanese Patent Application Publication No. 2006-186999